Line and command numbering in REPLs
A REPL session is a sequence of commands,
where each command is one or more input lines (usually typed by a human),
followed by the output of the result of evaluating the input lines.
The output can be zero over more text lines, but some envionments
allow printing
images, graphcs, or rich text
.
While typically each command is a single line,
sometimes commands may span over multiple lines.
This note discusses pragmatics of numbering those lines and commands,
and recommends that command numbers be identical to line numbers.
The most important reason for wanting command or line numbers is for error messages. Errors when compiling or loading a file generally have the form of a file name, followed by a line number (optionally a column-number or a range), possibly some identifying information (an error code or type), followed by a human-readable message. It makes sense to use the same form for REPL error messages. Run-time errors (or debugger break-points) may include a stack trace that references a similar file+line position.
Prompt number and eror messages
Suppose you want to work with hexadecial literals, which in Scheme have the prefix#x:
#|kawa:1|# #xff 255
(The default comment in Kawa Scheme has the form of a comment, to better support copy-and-paste. The color scheme will be discussed later.)
The next command spans two lines and has invalid charaters in the hex literal:
#|kawa:2|# (+ #xff pcont #xabcdefg) errloc: excess junk after number #|kawa:N|#
We have inter-related questions: What should be in the
continuation-line prompt pcont?
How should the location in the error message errlc?
Should the number of following command N
be 3 or 4?
When numbering commands
stdin2:2:10: excess junk after number
#|kawa:3|#
references to previous commands (history)
When numbering lines
First, my recommendation as implemented in Kawa, which is to number lines (rather than commands):
#|kawa:2|# (+ #xff #|.....3|# #xabcdefg) stdin:3:10: excess junk after number #|kawa:4|#
Smart input/output interleaving
Consistency with sourcing
a file
Display the output
In most IPython or Jupyter front-ends prefix
each input command prefixed with a prompt
In[number], but additionally
(first) output line is redundant prefixed with
Out[number],
which means less space is evailable for the output.
(Also, the output is indented, which may mess up
display of TAB characters, unless care is taken.)
Sometimes, for multi-line or graphics output
the Out[number] missing.
Better and more consistent is to always
leave off the Out[number];
it doesn't provide any extra information.
IPython/Jupyter front-ends also tend to leave quite a bit of whitespace between commands, which slightly improves visibility, but wastes valuable screen spaces. I suggest instead DomTerm use of a thin dashed line, which provides a clear demarkation between commands while using less space:
#|kawa:2|# (fill 'green (circle 30))IMAGE